As with road vehicles, there are two basic systems: electric drive and hybrid drive.
An electric drive has the propellor driven by an electric motor that is powered from a battery. The battery may be charged from a variety of sources including a small diesel generator or a fuel cell. A boat will probably have a choice of several methods.
A hybrid drive has an electric motor, powered from a battery and an internal combustion engine that are both able to drive the propellor. The motor, driven by the engine, can also be used as a generator to charge the battery. The battery may also be charged from shore power or solar panels.